Electric water heaters are used to heat and store a quantity of water in a storage tank for subsequent on-demand delivery to plumbing fixtures such as sinks, bathtubs and showers in both residences and commercial buildings. Electric water heaters typically utilize one or more electric resistance heating elements to supply heat to the tank-stored water under the control of a thermostat which monitors the temperature of the stored water.
An electric water heater is sold without water in its tank and is filled with water after it is moved to and installed in its intended operative location. The possibility exists that the water heater can be “dry fired,” i.e., have its electric resistance type heating elements energized before the storage tank is filled with water (thereby immersing the elements in the water) or otherwise in a condition in which the heating elements are not covered in water. When such dry firing occurs, the electric resistance heating elements may overheat, which may result in returning the unit to the manufacturer, or a service call by a repair technician to perform an on-site element replacement. As well, in those water heaters including bodies formed by plastic materials, damage to the body from excessive heat can render the water heater unrepairable.
Various solutions have previously been proposed to prevent energizing heating elements in electric water heaters unless the elements are immersed in water. These proposed solutions have taken two forms, float switch-based protective systems, in which the heating elements are activated only if a float sensor detects a water level in the tank above a certain level sufficient to cover the heating elements, and temperature sensor-based protective systems, in which the heating elements are activated only if a temperature sensor in contact with an outer surface of the water heater adjacent a corresponding heating element indicates a temperature below a predetermined threshold. Float switch-based systems, however, tend to be complex and costly to incorporate into the overall water heater assembly and include moving parts that can adversely affect reliability. Existing temperature sensor-based protective systems may be unreliable with regard to water heaters having tanks constructed of polymer materials, in that where the polymers are poor conductors of heat, damage may occur to the tank before the temperature sensor detects a dry fire condition.